This invention relates to a method for filling a plasma addressed liquid crystal display panel with liquid crystal material.
Three types of liquid crystal (LC) display panel which have been developed are the passive LC display panel, the active LC display panel and the plasma addressed LC display panel. The passive LC display panel and the active LC display panel each comprise two transparent plates in closely spaced parallel confronting relationship and a thin layer of liquid crystal material between the confronting faces of the plates. An addressing structure resolves the layer of liquid crystal material into a rectangular array of LC elements and applies an electric field across selected LC elements to influence the polarizing characteristics of the LC elements
The passive or active LC display panel is formed by securing the two transparent plates together with a bead of a suitable bonding material extending around the periphery of the plates, while leaving a gap in the bead at one corner of the panel. The confronting faces of the two transparent plates are separated by an LC space, which is currently empty of LC material. The panel is placed in a sealed chamber in a vertical orientation with the open corner of the panel directed downward. The chamber is evacuated, thereby also evacuating the space between the plates of the panel, and the open corner of the panel is dipped in a container of LC material. The partial vacuum in the chamber is then relieved, and LC material is forced into the LC space between the transparent plates. The gap in the bead is then sealed. Due to capillary action, the LC material spreads out between the plates and forms a layer of uniform thickness in the LC space.
A practical form of the plasma addressed liquid crystal (PALC) display panel is illustrated schematically in FIGS. 5 and 6 of the accompanying drawings.
The display panel shown in FIGS. 5 and 6 comprises, in sequence from below, a polarizer 2, a channel member 4, a cover sheet 6 (commonly known as a microsheet), a layer 10 of electro-optic material, an array of parallel transparent data drive electrodes (only one of which, designated 12, can be seen in the view shown in FIG. 5), an upper substrate 14 carrying the data drive electrodes, and an upper polarizer 16. The channel member 4 is typically made of glass and is formed with multiple parallel channels 20 in its upper main face. The channels 20, which are separated by ribs 22, are filled with an ionizable gas, such as helium. An anode 24 and a cathode 26 are provided in each of the channels 20. The channels 20 are orthogonal to the data drive electrodes and the region where a data drive electrode crosses a channel (when viewed perpendicularly to the panel) forms a discrete panel element 28. Each panel element can be considered to include elements of the layer 10 and the lower and upper polarizers 2 and 16. In the case of a color display panel, the panel elements include color filters (not shown) between the layer 10 and the upper substrate 14. The region of the upper surface of the display panel that bounds the panel element constitutes a single pixel 30 of the display panel.
When the anode in one of the channels is connected to ground and a suitable negative voltage is applied to the cathode in that channel, the gas in the channel forms a plasma which provides a conductive path at the lower surface of the cover sheet 6. If a data drive electrode is at ground potential, there is no significant electric field in the volume element of electro-optic material in the panel element at the crossing of the channel and the data drive electrode and the panel element is considered to be off, whereas if the data drive electrode is at a substantially different potential from ground, there is a substantial electric field in that volume element of electro-optic material and the panel element is considered to be on.
It will be assumed in the following description, without intending to limit the scope of the claims, that the lower polarizer 2 is a linear polarizer and that its plane of polarization can be arbitrarily designated as being at 0.degree. relative to a reference plane, that the upper polarizer 16 is a linear polarizer having its plane of polarization at 90.degree., and that the electro-optic material is a twisted nematic liquid crystal material which rotates the plane of polarization of linearly polarized light passing therethrough by an angle which is a function of the electric field in the liquid crystal material. When the panel element is off, the angle of rotation is 90.degree.; and when the panel element is on, the angle of rotation is zero.
The panel is illuminated from the underside by an extended light source (not shown) which emits unpolarized white light. A rear glass diffuser (not shown) having a scattering surface may be positioned between the light source and the panel in order to provide uniform illumination of the panel. The light that enters a given panel element from the source is linearly polarized at 0.degree. by the lower polarizer 2 and passes sequentially through the channel member 4, the channel 20, the cover sheet 6, and the volume element of the liquid crystal material toward the upper polarizer 16 and a viewer 32. If the panel element is off, the plane of polarization of linearly polarized light passing through the volume element of liquid crystal material is rotated through 90.degree., and therefore the plane of polarization of light incident on the upper polarizer element is at 90.degree.. The light is passed by the upper polarizer element and the pixel is illuminated. If, on the other hand, the panel element is on, the plane of polarization of the linearly polarized light is not changed on passing through the volume element of liquid crystal material. The plane of polarization of light incident on the upper polarizer element is at 0.degree. and therefore the light is blocked by the upper polarizer element and the pixel is dark. If the electric field in the volume element of liquid crystal material is intermediate the values associated with the panel element being off and on, light is passed by the upper polarizer element with an intensity which depends on the electric field, allowing a gray scale to be displayed.
In a practical implementation of the PALC display panel, the channel member 4 is etched back around the area in which the channels are formed in order to provide a plateau 34 in which the channels 20 are formed, and the cover sheet 6 is secured to the channel member by an endless frit bead 36 in a rabbet 40 extending around the periphery of the plateau. An upper substrate assembly, including the upper substrate 14 and the data drive electrodes 12 carried thereby, is attached to the channel member 4 by means of a glue bead 44 which extends almost completely around the frit bead but remains open at one corner. The confronting faces of the cover sheet 6 and the upper substrate assembly may be at a distance of about 5 .mu.m and are separated by an LC space.
The technique described above for introducing LC material into a passive or active LC display panel cannot be used with current PALC display panels because the pressure of the ionizable gas in the channels is significant, typically about 1/5 atmosphere. If the panel were placed in a vacuum chamber with the LC space open at one corner of the panel, and the chamber were evacuated to a sufficient degree, the force due to the pressure difference between the channels and the exterior of the panel could cause the panel to explode. Accordingly, a technique has been developed by which a fill tube is attached to the panel at the corner where the glue bead is open, and vacuum is applied through the fill tube only to the LC space. Spacers between the upper substrate and the cover sheet are sufficiently close together that the force due to the pressure difference across the cover sheet and across the upper substrate is not sufficient to damage the panel. When the LC space has been evacuated, the fill tube is connected to a source of liquid crystal material and the LC material is forced into the LC space. The LC material entering the LC space spreads out through the LC space in a front from the corner at which the glue bead is open and gradually fills the LC space by capillary action. When the LC space has been filled, the fill tube is sealed, thereby effectively sealing the gap in the glue bead, and the fill tube is disconnected from the sour e of LC material.
It has been found in the case of a moderate sized panel that it can take over an hour to introduce sufficient LC material to fill the LC space and provide a uniform layer of LC material throughout the LC space with this technique. During this time, the equipment used for introducing LC material into the LC space of the panel that is being processed is not available for processing another panel.
Commercially available liquid crystal materials have high resistivity, and it is important to operation of a PALC display panel that resistivity of the LC material not be impaired. One possible mechanism for reduction in resistivity of an LC material is ionic contamination. Ionic contamination can result from contact with various materials, such as certain sodium bearing glasses. Therefore, the materials with which the LC material comes into contact, both during the filling operation and after filling is complete, must be carefully selected in order to minimize the danger of ionic contamination.